UEFI vs BIOS for guest VM?

I am currently experimenting with Xen and have had about 85% success so far (for what I am doing I am very happy with this). The question I have is what is the difference between using a UEFI BIOS or a Legacy BIOS for booting guest VMs.

I know there are a ton of differences between UEFI and the legacy BIOS that touch on just about every aspect of a computers hardware, however when it comes to using virtual machines is there any advantage or difference between UEFI or BIOS from a performance or ease of use?

Can one handle hardware (VGA) passthrough better than the other?

The only major thing i have noticed is that Xen does not yet seem to support UEFI.

Thanks for any insight!